The present invention relates to electro-optics in general, and more particularly to photoacoustic material analysis.
Conventional methods of material analysis such as absorption and luminescent spectroscopy, Raman spectroscopy, and measuring polarization and reflectance changes are not sufficiently suitable for a turbid medium such as human tissue due to significant diffuse scattering of the reference light beam. As an alternative, other material analysis techniques employ photoacoustic spectroscopy, in which a laser beam is used to rapidly heat a sample generating an acoustic pressure wave that can be measured by high-sensitivity ultrasonic detectors such as piezo-electric crystals, microphones, optical fiber sensors, laser interferometers or diffraction sensors.
The laser radiation wavelength is selected so as to be absorbed by the interest component in the medium being analyzed, Thus, laser excitation of a medium is used to generate an acoustic response and a spectrum as the laser is tuned. The use of photoacoustic spectroscopy for glucose testing in blood and human tissue can provide greater sensitivity than conventional spectroscopy. An excellent correlation between the photo-acoustic signal and blood glucose levels has been demonstrated on index fingers of both healthy and diabetic patients.
A prior art method and apparatus for noninvasive measurement of blood glucose by photo-acoustic techniques is described in U.S. Pat. Nos. 5,941,821 and 6,049,728, in which an excitation source provides electromagnetic energy at a wavelength corresponding to the absorption characteristics of the analysis. Upon irradiation, acoustic energy is generated in a relatively thin layer of the sample to be measured, characterized by a heat-diffusing length. The acoustic emission is detected with a differential microphone, one end of which is positioned in a measuring cell and the other end of which is positioned in a reference cell. A processor determines the concentration of the substance being measured based upon the detected acoustic signal. In order to determine the concentration of glucose in the bloodstream, the excitation source is preferably tuned to the absorption bands of glucose in spectral ranges from about 1520-1850 nm and about 2050-2340 nm to induce a strong photo-acoustic emission. In these wavelength ranges, water absorption is relatively weak and glucose absorption is relatively strong. Thus, even though tissue may have a high percentage of water at the above-specified wavelength ranges, the electromagnetic radiation is able to penetrate through the tissue to a sufficient depth to allow for accurate measurements. Despite water absorption, the acoustic signal which is generated by the absorption of electromagnetic radiation by glucose is not overwhelmed by that generated by water. The glucose optically absorbs the energy inducing a temperature rise and generating an acoustic emission indirectly in the air. Thus, the photo-acoustic intensity is approximately linearly proportional to the glucose concentration.
Unfortunately, prior art photo-acoustic material analysis techniques are disadvantageous in that they teach the application of energy to a medium without giving consideration to its acoustic oscillation properties, thus requiring relatively high laser power. Consequently, such techniques are energy inefficient, and provide an inadequate level of sensitivity.
The primary object of the present invention is to provide a novel method and apparatus of resonant photoacoustic spectroscopy (RPAS) for material analysis that is suitable for determining a concentration of an interest component in a medium. The method comprises irradiating a surface of the medium having the interest component with a light pulse-train comprising equidistant short pulses having variable duration, frequency, number, and power. The frequency of the light short pulses is chosen equal to a natural acoustic oscillation frequency in a medium for resonant light-excitation of acoustic modes. The wavelength of the light pulses is selected so as to excite resonant acoustic oscillation in the medium due to absorption of light by the interest component and subsequent adiabatic temperature rise in the testing area of the medium.
Another object of the present invention is to provide a novel method and apparatus for determination of the concentration of an interest component in the medium like human tissue. A pulsed laser beam generates acoustic oscillations in the dermal or epidermal area of the skin that can be considered as a thin membrane. The membrane has the natural frequencies of acoustic oscillations that depend on the elastic constants of the membrane and its thickness and square. According to the present invention, if the frequency repetition of the light short pulses in the pulse-train equals the natural oscillation frequency of the membrane, resonance of acoustic oscillation results. The amplitude and frequency of the resonant acoustic oscillations depend on the concentration of interest component in the human tissue due to absorption of light with a predetermined wavelength. The concentration is determined in response to electrical signals of a detector of the resonant acoustic oscillations exciting in the medium. The present invention is suitable for measuring blood components in human tissue, especially glucose.
A further object of the present invention is to provide a novel type of the Q-switched laser device for material analysis, which is suitable for noninvasive blood glucose monitoring, based on the RPAS as disclosed herein. The laser device is preferably a solid-state laser with an unstable resonator and passive Q-switch (PQS) on colored LiF crystal. The transparency of the PQS changes linearly with crystal length that provides pulse-train generation giving equidistant short pulses having variable duration, frequency, number and power. It is possible to optimize frequency and energy of laser radiation by moving the PQS to a position perpendicular to the optical axis of the laser cavity.